The monitoring of soot generated by combustion processes in machinery, in vehicles, generators, engines, boilers, and the like (all hereinafter, for convenience, generically referred to as “machinery”) is vital to ensuring compliance with environmental regulations on particulate matter (PM) emissions. In the United States, the environmental protection agency (EPA) has mandated reductions in particulate matter emissions from diesel engines, referred to as PM10 and PM2.5 (which refers to fine particles less than 10 microns and 2.5 microns in size, respectively.) Inhalation of fine particles is associated with serious health effects, and pollution such as smog. There is no single sensor available that can adequately monitor soot in-situ, as prior art technology is subject to fouling, susceptible to vibrations, insufficiently sensitive, or non-specific. Prior art soot sensor technology includes U.S. patent application Ser. No. 11/770,396 filed Jun. 28, 2007, which discloses a soot sensor representing the image charge method of measurement. The sensor design is similar to a spark plug. Soot particles in the exhaust, a fraction of which are charged, reduce the breakdown voltage of the spark gap. A second type of soot sensor is disclosed in U.S. patent application Ser. No. 11/827,029, filed Jul. 10. 2007. This method measures the mass of soot accumulated on a quartz microbalance placed in the exhaust stream. A resistive heater is manufactured on top of the resonant element to oxidize accumulated soot. Microbalances respond to any material or liquid absorbed on the surface of the resonator, and are, in this sense, non-specific to soot. A third category of particulate matter sensors are systems using optical scattering to determine the size and concentration of airborne particles. The optical method is too expensive for automotive applications, and secondly is subject to rapid fouling caused by soot accumulation on the optical components. The present invention differs from these approaches in a fundamental way: namely, a solid-state sensor that offers direct in situ monitoring by microwave ESR spectrometry of paramagnetic resonance signals that are characteristic of soot and other carbonaceous products of combustion.
Most diesel engine particulate emissions reduction systems are passive diesel particulate filters mounted on the engine exhaust. Soot particulates accumulate in a ceramic filter. Once the filter reaches capacity, it can be regenerated by burning off the accumulated soot. However, a different approach is to adjust the engine fuel injector timing sequence to reduce the overall quantity of soot emitted from the engine. A soot sensor, analogous to oxygen sensors used in gasoline engines, is therefore needed to implement feedback servo control of the fuel injection parameters. There are to date, however, no commercially-available vehicle mounted sensors that provide a repeatable, linear, real-time monitoring of the soot particulates generated by combustion.
This present disclosure reveals a new, low-cost, and highly specific electronic method for monitoring soot particulates in a vehicle exhaust stream. The sensor measures the electron spin resonance (also called electron paramagnetic resonance or simply paramagnetic resonance) properties of the carbonaceous products of combustion (See Literature Reference No. 1). Electron spin resonance is a microwave absorption phenomenon unique to paramagnetic substances, including several forms of carbon (See Literature Reference Nos. 2 and 3). As such, the method is sensitive and highly specific to soot.
In the present application, novel, miniaturized ESR spectrometers are disclosed for such direct sensing of soot particulates in an exhaust stream. The structure discloses a thread-in “bolt plug”-type ESR sensor suited to a high-temperature environment. This design can be mounted in a vehicle exhaust stream with minimal changes to the vehicle. Input and output channels are provided for passing the exhaust stream through the sample chamber at the center of the sensor. The present invention is robust to vibrations and fouling.
Prior related applications include U.S. application Ser. No. 11/590,522, filed Oct. 31, 2006, entitled “Method Of And Apparatus For In-Situ Measurement of Degradation of Automotive Fluids And The Like By Micro-Electron Spin Resonance (ESR) Spectrometry,” and related Continuation in Part Application U.S. Ser. No. 11/983,393, filed Nov. 8, 2007, “Method of And Apparatus for In-Situ Measurement of Changes in Fluid Composition By Electron Spin Resonance (ESR) Spectrometry.” These disclosures provide detailed background on the use of miniaturized, in-situ ESR sensors for measuring the properties of lubricating oils and other fluids. Neither the above referenced applications nor this present application, however, involves the first use of an ESR spectrometer, though they are believed to be the first adapted and described for the purpose of the specific invention—in the present application, the first miniaturized ESR sensor adapted to monitoring carbonaceous particulates during engine operation.
Paramagnetic Resonance of Soot
The paramagnetic resonance spectrum of soot particulates is well known in the scientific literature. In particular, the study by C. Yamanaka, T. Matsuda, and M. Ikeya, entitled “Electron spin resonance of particulate soot samples from automobiles to help environmental studies,” published in 2005 (See Literature Reference No. 4), and an earlier study by M. M. Ross et al., “Electron Paramagnetic Resonance Spectrometry of Diesel Particulate Matter,” published in 1982 (See Literature Reference No. 7), are directly relevant to the present invention. These articles show examples of the electron spin resonance spectrum of diesel particulate emissions. The diesel particulate spectrum has two components: a broad resonance line at g=2.1 with a line width of 80-120 mT, and a narrow resonance signal at g=2.003 with a line width of 0.4 mT (See Literature Reference No. 4). These two ESR signals respond in distinct fashion to atmospheric pressure and heat treatment (See Literature Reference No. 4 and 6). The mechanisms for the changes in the ESR spectra under vacuum, and after heat treatment, are further elucidated in Carbon, Volume 37, 1741-1747, (1999) (See Literature Reference No. 12), a detailed study of commercially available carbon black samples. High purity carbon black is also characterized by a broad and a narrow ESR signal. Additional examples of electron spin resonance studies of related carbon materials are given in articles references in Literature Reference Nos. 2 through 13, which incorporated herein by reference.
Prior ESR Spectrometers in General
Microwave electron spin resonance spectrometers of a myriad of types have heretofore been developed for uses other than that of the present invention. U.S. Pat. No. 4,803,624 issued Feb. 7, 1989, for example, discloses an electron spin resonance spectrometer operating at frequencies in the range of 2 to 3 GHz, using loop-gap resonators at these frequencies in a preferred embodiment. This spectrometer uses a circulator to measure the reflected microwave power from the resonator, the same as in most commercially available electron spin resonance spectrometers. Microwave circuit components, for example an isolator, circulator, power dividers, variable attenuator, and directional couplers, are arranged in a microwave bridge connected by microstrip transmission lines. External components, such as the microwave source and loop-gap resonator, are connected via SMA coaxial connectors. The microwave circuit construction uses microstrip transmission line connections formed by RF circuit boards laminated onto an aluminum backplane. This patent suggests the use of Sm—Co based permanent magnets and auxiliary field sweep coils, but does not present detailed embodiments of the magnet.
Another prior art microwave electron spin resonance spectrometer is disclosed in U.S. Pat. No. 5,233,303, issued Aug. 3, 1993. The spectrometer operates in the 2 GHz frequency range, and is intended for portable use. The design similarly uses a circulator to measure reflections from the microwave resonator containing the sample, lock-in detection, and computer control. The resonator and sample chamber is a split-ring resonator formed by plating 1-5 microns of silver onto a quartz tube. The permanent magnet design consists of an open U-shaped yoke with rectangular cross-section, two opposing cylindrical permanent magnets with amorphous iron pole pieces (e.g. Metglas), and copper wound coils to provide a modulated magnetic field ramp.
U.S. Pat. No. 4,888,554 issued Dec. 19, 1989 discloses an electron spin resonance spectrometer that detects both the absorption and dispersion signals caused by magnetic resonance, by using in phase (I) and quadrature (Q) mixers. The preferred embodiment uses a microwave circulator connected to the resonant cavity; for example, a loop-gap resonator. An automatic frequency control loop (AFC) is disclosed to servo the microwave source to the cavity resonant frequency.
Other prior art electron spin resonance spectrometers for other purposes than the present invention include U.S. Pat. No. 5,142,232 issued Aug. 25, 1992, U.S. Pat. No. 5,389,878 issued Feb. 14, 1995, and U.S. Pat. No. 5,465,047 issued Nov. 7, 1995. U.S. Pat. No. 5,142,232 discloses a spectrometer design intended to provide an inexpensive ESR system with reduced weight. A permanent magnet is provided with a moveable yoke for adjustment of the magnet field. One pair of permanent magnets is attached to a stationary yoke, and a second, moveable yoke in a parallel magnetic circuit provides mechanical adjustment of the field. Carrier suppression techniques are shown in U.S. Pat. No. 5,389,878 to reduce the carrier power reflected from the resonator, which may improve spectrometer sensitivity, depending on the noise properties of the microwave source. U.S. Pat. No. 5,465,047 shows yet another ESR spectrometer, which uses frequency sweep of the microwave source and resonator, and a fixed permanent magnet. The tunable resonator described in U.S. Pat. No. 5,465,047 is a cylindrical waveguide cavity resonator with a moveable end plate for frequency adjustment. The resonator end plate is driven by a motor.
Microwave Cavities for Prior Art ESR—Structures and Usages:
Eddy-current shielding of the audio frequency modulation field is well known in the art of electron spin resonance, and typically requires special construction techniques for the cavity design. U.S. Pat. No. 5,596,276 issued Jan. 21, 1997 uses non-uniform metal thicknesses in the construction of a rectangular waveguide cavity to reduce eddy current shielding by the metal surfaces. More commonly, thin layers of electroplated metal are used to define the microwave resonator surfaces, while providing minimal shielding of audio frequency fields. An exemplary method for building a loop-gap resonator, disclosed in U.S. Pat. No. 4,435,680 issued Mar. 6, 1984, is to machine the resonator elements from MACOR® ceramic, deposit a conductive seed layer by a chemical silvering process, and electroplate silver or copper onto the seed layer to a thickness of several microns.
Several types of apparatus have been used for handling fluids in electron spin resonance experiments. Dielectric loss is of particular importance for liquid samples containing water and requires special techniques. One type of cavity adapted to aqueous samples is shown in U.S. Pat. No. 3,931,569 issued Jan. 6, 1976. Another type of cavity with a fluid handling apparatus is disclosed in U.S. application Ser. No. 10/197,236, filed Jul. 15, 2002 and another is said parent application.
The novel ESR microwave system structures of the present invention, unlike the prior art, are specifically designed for the purposes and objectives of the invention; in the present case microwave ESR cavity systems applied to direct measurement of soot in the exhaust stream of machinery.